Agreed... And obligatory origins are boring as fawk.
I think Nolan and Co. are more concerned with character development as opposed to "origins"-- and the character to have evolve from film to film is that of Bruce Wayne.
I mean, the premise to Batman Begins is sort of flimsy, no? (I'm not Nolan-bashing: hear me out for a second) I mean, Bruce Wayne's principal intentions are rather naive, reflecting that of a rich kid becoming barely politically aware (Katie Holmes takes him on a consciousness-elevating joyride through the ghetto, and Bruce concludes "I'm going to show the people of Gotham that they don't have to fear the corrupt." C'mon!). He wants to rid Gotham of the systemic, organized criminal element that (somehow) renders much of Gotham into a violent, impoverished Kowloon-esque ghetto. Violence is a tool for the mob, not an end. So, if Batman can bring down the mob and scare civic officials straight, then law and order will be restored. Violence goes bye-bye. Mom and apple pie ensue.
The Joker, on the other hand, does not share the mob's motivations. The Joker's violence is wholly irrational -- representing something much darker, and more difficult to deconstruct. I think it's clear that Nolan is pretty interested in going in this direction, given the teaser trailer ("Some people just want to watch the world burn") -- and I hope that Nolan's Bruce Wayne will mature a bit in the process. I mean, think of Harvey Dent/Two-Face: he is just as violent as the Joker, and just as irrational. The Joker represents the taboo, violent impulses of the unconscious; Two-Face represents the arbitrary, unjust working of a godless universe (i.e. a coin-toss).
I just don't think it'll service Nolan's Batman if he understands that the Joker was thrown into a vat of acid, beaten as a child, grew up in too close of a proximity to power-lines, etc. After all, we don't know the "origins" of our own dark, libidinal impulses...